Applications of low band gap polymers in solar cells have attracted intense attention due to their energetic overlap with the solar spectrum. Recently, low band gap organic photovoltaic (OPV) materials have shown an unprecedented 8% efficiency in solar cell devices. Although the energetic alignment is crucial in the optimization of these materials, the structural and kinetic effects are also important factors in the overall device performance. Here we focus on the morphology and charge separation kinetics of several energetically similar low band gap materials. Special emphasis will be on two polymers, PF and PTB, in this report.